Electrophotographic marking is a well known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a substantially uniformly charged photoreceptor with a light image representation of a desired document. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image to form a toner image. That toner image is then transferred from the photoreceptor onto a copy substrate, such as a sheet of paper. The transferred toner image is then fused to the copy substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a black and white electrophotographic marking machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, called the REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner to produce a black toner image. The recharge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various latent images and consequently the color toners are placed in a superimposed registration such that a desired composite color image results. That composite color image is then transferred and fused onto a substrate.
The foregoing color printing process can be performed in a various ways. For example, in a single pass printer wherein a composite image is produced in a single pass of the photoreceptor through the machine. This requires a charging, an exposing, and a developing station for each color of toner. Single pass printers are advantageous in that they are relatively fast since a composite color image can be produced in one cycle of the photoreceptor.
One method of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is typically comprised of a laser light source (or sources), a rotating polygon having a plurality of mirrored facets, and pre-polygon and post-polygon optical systems. The light source radiates a laser beam into the pre-polygon optical system. The optical system collimates the laser beam and directs the collimated beam onto the rotating polygon facets. Those facets reflect the incoming beam into a sweeping beam that is directed into the post-polygon optical system. The post-polygon optical system corrects for various defects (such as wobble correction and scan line non-linearities) and focuses the sweeping beam onto a photoreceptor, thereby producing a light spot. As the polygon rotates the spot traces lines, referred to as scan lines, on the photoreceptor. By moving the photoreceptor in a process direction (also referred to as the slow scan direction) as the spot traces scan lines in the fast scan direction, the surface of the photoreceptor is raster scanned by the spot. During scanning, the laser beam is modulated by image data synchronized with the movement of the spot across the photoreceptor. Thus, individual picture elements ("pixels") of the image are sequentially created on the photoreceptor.
While raster output scanners are beneficial, they have problems. One set of problems relates to scan line position errors in the slow scan direction. Scan line position errors of greater than 10% of the nominal line spacing can be noticeable in a half tone or continuous tone image. Because of the sensitivity of the human eye to color variations, color images are even more susceptible to scan line position errors.
Scan line position errors arise from many sources, such as polygon and/or photoreceptor motion flaws, facet and/or photoreceptor surface defects, photoreceptor stretching, and phasing errors between photoreceptor motion and facet position. Phasing errors arise because when the photoreceptor is in the proper position to receive an image a facet may not be in position to produce a scan line. As the printer delays writing a scan line until a facet is properly positioned the photoreceptor continues advancing. When a facet is properly positioned the photoreceptor has advanced, producing a scan line error. While phasing errors are generally small, in high quality systems, particularly color, the errors can be noticeable.
Scan line position errors can be corrected using closely spaced light valves (such as liquid crystal modulators, reflecting Fabry-Perot modulators, total internal reflective modulators, or a waveguide modulator/amplifier) that selectively block portions of a light beam from reaching the photoreceptor. Reference U.S. Pat. No. 5,049,897 issued on Sep. 17, 1991 to Ng entitled "Method and Apparatus for Beam Displacement in a Light Beam Scanner," and U.S. Pat. No. 5,764,273, issued on Jun. 9, 1998 to Paoli entitled, "Spot Position Control Using a Linear Array of Light Valves."
The use of closely spaced light valves to selectively block portions of a light beam is a useful technique since the position of a scan line on a photoreceptor is directly controlled by selecting which light valve(s) should pass light. That technique is particularly beneficial for correcting for phasing errors. Unfortunately, prior art techniques of selecting which light value(s) to turn on require the determination of the existence and the extent of scan line position errors. Only then can the proper light valve(s) be selected. U.S. Pat. No. 5,764,273 teaches using a feedback control system comprised of marks on the photoreceptor, a synchronization strobe and sensor, a signal processing circuit, a control apparatus, and a switching circuit that selects the proper light valves. Alternatively, U.S. Pat. No. 5,764,273 teaches using stored data and a switching circuit. U.S. Pat. No. 5,049,897 teaches using an encoder that monitors the web (photoreceptor) speed, phase-locking the raster output polygon motor to the web (photoreceptor), logic circuitry that compares the web (photoreceptor) speed with a predetermined constant, a logic and control unit (LCU) that calculates a potential scan line spacing error and that generates a control signal, and a driver that uses the control signal to select the proper light valve(s) to pass light.
While the prior art techniques of selecting the proper light valve(s) to correct for slow scan spot position errors are beneficial, they are rather complex, costly and/or difficult to implement. This is particularly true when correcting for phasing errors. Thus, a need exists for an improved method of determining which light valve(s) should be selected so as to correct for slow-scan direction spot position errors. Even more beneficial would be a simple, easily implemented method of selecting the proper light valve(s) to pass light when correcting for phasing errors.